The best age is when you are physically, mentally and emotionally ready. But if you don't consider the stability part, the best will be before 30. You will be able to enjoy your child's childhood better. I mean if you have child at 40-45 by the time your child goes university you will be in your 60s. While you have a child at 25-30 you will be still youth and will be able to connect to their generation better.